Brian also (unwittingly) influenced the power section of this amp. A while back he picked up the power and reverb amps from a 1969(?) Hammond organ and had them shipped to me so I could convert the reverb amp for guitar usage. My "payment" was that I got to keep the push-pull EL84 power amp and three speakers, so that got me thinking about building my first EL84 amp.
The EL84 idea got me researching other EL84 amps, where I picked up the rest of the pieces that went into the Verberator. The input stages are lifted from the Vox AC15 (EF86 pentode) and the Matchless Spitfire (parallel triodes), and the power section is pretty much the same as a Marshall 18-Watt.
OK, so here's the topology: the guitar input goes to an EF86 pentode and a dual triode with the two triodes in parallel (at the moment, it's a 12AY7). Both of these go through their own gain pots, which feed a cathode follower stage. Next there's a Baxandall tone stack (a feature of my Ampeg Gemini I that I've always liked) and then another triode to get the signal strong enough to split between the dry and reverberated signal paths. The reverb signal goes through a pretty standard Fender-style circuit (a paralleled 12AT7 pushing the tank and 1/2 12AX7 handling the recovery), but with the Dwell (i.e., reverb gain) and Tone controls in addition to the usual Mix. Then we have a master volume control and one last triode stage before the PI and EL84s.
For more detail, check out the
I started this project by building a prototype of most of the preamp (including the reverb) that slides into the top slot of my Tonerator head. Except for the reverb, the proto doesn't really match what I eventually built, but it sounded great, and that gave me the courage to forge ahead and keep adding features.
The circuit wasn't the only part of this project that was new ground for me. This was my first full-size combo, so I had to deal with new issues regarding construction. The most challenging were the size of the chassis (and the weight it had to bear), a presentable control panel, and the cabinet to put it all in.
As I mentioned above, I really wanted to do as much as possible on this project by myself, but metalworking is not an area in which I have much experience or tooling. At work I have access to a roller/cutter/brake that can handle 20 ga. cold-rolled carbon steel, so that put a limit on the chassis material.
I'd used 22 ga. steel to make the Decimator chassis, and thought it would do for the Verberator, so I cut and bent a 22 ga. chassis as soon as I figured out how big I wanted it to be. It immediately became obvious that it wasn't going to handle several pounds of transformers very well, even if I kept them near the relatively strong edges.
I didn't find any good sources for 20 ga. steel, but eventually found that McMaster Carr carries .0312" sheets - not quite 20 ga., but thicker than 22 ga., so I decided to try it. The new chassis still flexes a little bit with everything on it, but when the ends are supported by the 'rails' in the cabinet the largest surface seems rigid enough. Adding dowels across the top corners stiffened up the whole thing quite a bit, so now I'm pretty confident that I've got something that'll last a few years...
The other difficult part about the chassis was making it look good. My other home-brews have unmarked control panels, which is fine when there were only a few controls, but I really wanted this one to look more professional. I've since learned that it's possible to get laser-engraved faceplates for very reasonable prices, but at the time the best I could think of was to paint the chassis and use press-on lettering.
In this photo you can see the lettering well
enough to get the idea. There are several coats of flat black paint, the press-on lettering, and then a couple
of coats of a clear satin protective finish.
The cabinet construction was also a challenge. As with the chassis, I wanted to make it look better than my previous attempts, and there was also the challenge of coming up with a design suitable for a full-size combo. The original idea was a stained clear pine cabinet, but I quickly found out that it was going to be difficult to find clear pine that wide, and it would probably cost me a fortune if I did!
As luck would have it, I came across some very nice clear fir just when I was giving up on pine. I thought I'd be staining that, too, but I found the grain very attractive. An overnight test (stain on one scrap, polyurethane on another) convinced me that it would be a crime to do much more than protect what was already there.
The final challenge was to put it together without having screw heads showing all over the place. I knew I could dowel all the cross pieces to the two sides (and plug the dowel holes), but I was a little slow figuring out a scheme that would let me mount the speaker baffle from the rear. Once I saw that two cross pieces were all I needed (i.e., no side attachments necessary), it practically put itself together!
This is how it looks when I'm tweaking. The stand for holding the chassis while I'm working on it was well worth the few minutes it took to slap it together.
This shot also gives you a pretty good idea of the construction of the cabinet. The chassis just slides in on those wooden rails, and is held in place by the cross piece on the front of the cabinet and the back panel (see above).
I didn't really plan the layout in advance, so I have a ridiculous number of spare eyelets on this board. What the heck – maybe some day I'll get tired of it and build a whole different circuit in there...
This is a close-up of the speaker and line out jacks. That big gold unit in the top left corner is the 8 ohm 50 watt resistor that guarantees I've always got a load (so I can use the line out without having a speaker running, too).